1. Field of the Invention
This invention relates in general to methods and apparatus to measure and record data related to the draft of a water-borne vessel and the amount of cargo in the vessel.
2. Prior Art
Water-borne vessels, such as ships and barges, are commonly used to transport cargo utilizing oceans, navigable rivers, canals and lakes. Their ability to carry large amounts of cargo economically makes them particularly suitable for transporting dry bulk cargo, liquid cargo and other similar type cargo that can be loaded by clamshell buckets, conveyors or pumped into the vessel holds. However, to improve the transportation economics, as well as to be able to determine the amount of cargo that has been loaded onto the vessel, it is important to obtain accurate readings of the vessel draft. It is also important to have accurate vessel draft readings to prevent the barge from bottoming out in the waterway during the transporting of the cargo.
Particular problems exist with current barge drafting measurements. In the current system, barge drafters use modified tape measures to obtain a freeboard height using a theoretical distance from the top of the barge to the water level at six set locations around a barge. According to the Barge Surveying Taskforce, a group of individuals put together by the Fertilizer Institute to standardize barge drafting measurements, variations among readings exceed ±3.5% and are seldom repeatable within 0.5% which can lead to a cost variation of over $10,000 per barge.
Freeboard measurements are an attempt to measure the distance from the molded deck down to the water line. Each barge generally has a published depth, which is measured from the deck of the barge to the bottom of the barge. However, during the lifetime of the barge, the decks generally become uneven due to stresses and collisions, making it difficult to find the “true” plane of the barge deck. While there are many problems with the current manual system of barge drafting, most errors can be traced back to one of four main areas. These include inaccurate integration of the measurements to the barge, failure to accurately estimate where the “true” water level due to wave activity, data entry and computational errors resulting from the method of recording the measurements made.
Due to effects that collisions, stress and other factors have on the barge deck, it is difficult to accurately locate the top of the barge for measuring the distance to the top of the water. However, each barge generally has a series of draft numbers permanently attached to the side of the barge adjacent the four corners of a barge. Each block number has a height of exactly six inches and is calibrated from the bottom of the barge. These numbers generally are not affected by stress or collisions. Surveyors use these numbers to establish a zero point where the “true” top of the deck should be located. The use of these numbers as a zero point allows integration into the barge dimensions regardless of the condition of the deck. Other barge drafting systems do not provide for direct integration into the barge draft numbers and work on the assumption that the molded deck has no abnormalities. This assumption is rarely true and the error introduced can throw the calculations off by a number of inches.
The draft of the barge is the distance from the top of the barge deck to the level of the water in which the barge is floating. It is important to establish an accurate water level regardless of the wave activity surrounding a barge. Currently surveyors drop a tape measure down to the water and estimate when the tape's end point is halfway between the peak and valley of the waves intersecting the barge. However, this is simply a visual estimate and can vary a number of inches depending on the surveyor and wave conditions. Conditions typically produce waves in excess of one foot down to an inch in amplitude. Conditions on oceans and rivers can drastically affect the freeboard measurements when compared to readings taken within an area such as a protected port. Furthermore the exact point along the curve of the wave being measured is currently determined by the surveyor which leads to differences amongst surveyor practices. In addition, the poor visibility at the time the measurement is taken can also create problems with the measurement. This is particularly true if one is measuring vessels, such as barges, that are tied up next to one another. In such instances there may only be one inch separating the adjacent vessels which impairs the surveyor's ability to make visual observations regarding the position of the tape measure. Additionally, such measurements can be dangerous if the vessels are rocking due to wave action or other forces. The marine surveyor can slip and fall from the vessel, or in some cases his hand, foot or other body part can get crushed between two vessels that rock into one another.
Surveyors measure the barge drafts at the location of the vessel as it is floating. In many cases, the location of the vessel is an isolated area. The surveyors currently manually write down their freeboard values taken from their tape measure observations. These values are later used for computation and then finally for the final survey report. Thus additional problems occur because of the double entry of the measured distances.
One attempt to obtain more accurate barge draft readings was through positioning of pressure sensors below the water surface of the barge. One such device is described in U.S. Pat. No. 5,547,327 entitled “Method and Apparatus for Continuously Determining the Inclination and Draft of a Waterborne Floating Vessel to Enable Automatic Loading of the Vessel” that issued on Aug. 20, 1996. However, this solution has not found widespread commercial acceptance. Difficulties in positioning the pressure sensors, taking accurate reading of the sensors, analyzing the sensor readings, and the potential of damage to the sensors during transportation are suspected difficulties that still leave the need for more accurate and reliable methods and apparatus to measure barge draft and the amount of cargo loaded in a barge.
Another attempt to solve these industry problems is the use of multiple ultrasonic sensors to determine the barge draft. One such device is described in U.S. Pat. No. 6,836,746 entitled “Method and Apparatus for Calculating the Payload on a Water-Borne Vessel” and issued on Dec. 28, 2004. However, this solution has also not found widespread commercial acceptance. Again difficulties in positioning such sensors and obtaining accurate readings when the vessel is rocking or measuring the draft of vessels that are tied up next to one another where there may only be one inch separating the adjacent vessels still leaves a need in the industry for better methods and apparatus to determine vessel drafts.